1. Field of the Invention
The present invention concerns a method for homogenization of the B1 field for a magnetic resonance data acquisition in at least one specific region of an examination volume in the magnetic resonance system. The invention concerns a corresponding magnetic resonance system with which such a method can be implemented.
2. Description of the Prior Art
Magnetic resonance tomography has become a widespread technique for the acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this modality the body or a body part of the patient to be examined must initially be exposed to an optimally homogenous static basic magnetic field (usually designated as the B0 field), which is generated by a basic field magnet of the magnetic resonance measurement device. During the acquisition of the magnetic resonance images, rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding of the magnetic resonance signals. With a radio-frequency antenna, RF pulses of a defined field strength are radiated into the examination volume in which the examination subject is located. The magnetic flux density of these RF pulses typically is designated B1. The pulse-shaped radio-frequency field therefore is generally called a B1 field for short. By means of these RF pulses, the nuclear spins of the atoms in the examination subject are excited such that they are moved from their state of equilibrium, which was parallel to the basic magnetic field B0, by what is known as an “excitation flip angle (called “flip angle” in the following). The nuclear spins then precess in the direction of the basic magnetic field B0. The magnetic resonance signals thereby generated are acquired by radio-frequency receiving antennas. The receiving antennas can be either the same antennas with which the RF pulses were radiated, or separate receiving antennas. The magnetic resonance images of the examination subject are ultimately created based on the received magnetic resonance signals. Each image point in the magnetic resonance image is associated with a small body volume, known as a “voxel”, and the brightness or intensity value of each image point is linked with the signal amplitude of the magnetic resonance signal received from this voxel. The relationship between a resonant radiated RF pulse with a field strength B1 and the flip angle a produced thereby is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic ratio that can be considered as a fixed material constant for most magnetic resonance examinations, and τ is the effective duration of the radio-frequency pulse. The flip angle achieved by an emitted RF pulse, and thus the strength of the magnetic resonance signal, consequently also depends on (aside from the duration of the RF pulse) the strength of the radiated B1 field. Spatial fluctuations in the field strength of the excited B1 field therefore lead to an inhomogeneous irradiation of the examined volume and correspondingly—depending on the complexity of the imaging sequence used—to an insufficient image quality. This can cause certain MR procedure, such as functional imaging or spectroscopy to be impossible.
Given high magnetic field strengths—that are inevitably present due to the necessary magnetic base field B0 in a magnetic resonance tomograph—the RF pulses disadvantageously exhibit an inhomogeneous penetration behavior in conductive and dielectric media such as, for example, tissue. This leads to the B1 field being able to significantly vary within the measurement volume. This effect becomes stronger with increasing B0 field strength and with higher frequencies of the B1 field.
The use of conventional methods for construction and tuning of RF antennas in typical magnetic resonance systems with B1 field strengths of one to two Tesla essentially effect only a homogenization of the antenna profile of the unloaded antenna. This means that the distorting influence of the human body on the B1 field is not taken into account. There exist static methods that optimize the homogeneity of the RF antenna dependent on the load situation, but these must not and cannot be adapted in operation. In modern magnetic resonance systems with high basic magnetic field strength of three Tesla (known as ultra high-field magnetic resonance examinations), however, it is no longer sufficient to ensure only that the unloaded RF antenna generates a homogenous B1 field. Rather, the B1 field must be “pre-distorted” in a suitable manner in order to obtain an optimally homogenous irradiation in the case of loading by a body (or body part) to be examined.
Recently, various technical and physical measures have become known in order to at least partially restore the homogeneity of the B1 distribution in specific regions of the human body given high-field magnetic resonance examinations. For example, in Proc. Intl. Soc. Mag. Reson. Med. 9 (2001) 1096 it is proposed by Yang et al. to compensate distortions of the B1 field in the body of the patient with by using pillows filled with water and positioned on the patient.
A further promising approach is specified in German OS 101 24 465. In this document, a transmission and reception coil for MR apparatuses is specified which is composed of a number of individual antenna elements (resonator segments) that are arranged around the examination volume within a gradient tube. These antenna elements are interconnected to form a large-surface volume antenna, similar to a type of antenna known as a birdcage antenna. The individual antenna elements are electromagnetically decoupled from one another by interconnected capacitors. A separate transmission channel via which the radio frequency feed ensues is associated with each antenna element. Phase and amplitude thus can be individually predetermined for each antenna element. In principle, this enables a complete monitoring of the radio-frequency field distribution in the examination volume (known as “RF shimming”). It is proposed to improve the homogeneity of the RF field in the entire examination volume in this manner.
A very similar method is specified in U.S. Application Publication No. 2003/0184293. In this document it is also proposed to assemble the antenna from similar, separately activatable elements and to improve the homogeneity of the RF field in the entire examination volume by a corresponding activation of the individual antenna elements.
As explained above, however, the B1 inhomogeneities trace back in large part to anatomically-dependent properties of the body to be examined. Consequently, the occurence is individual and position-dependent. Thus the goal of the homogenization is posed differently from case to case. Moreover, the known methods cited above have no general validity, such that they are not applicable in each of the cases. The selection of the matching methods for homogenization of the B1 field in a specific situation case requires from the operator of the magnetic resonance system a significant degree of experience and specialized knowledge, both in a physical respect and in a biological or medical respect, and is additionally extremely time-consuming since often a number of homogenization attempts are necessary until a homogeneity sufficient for a subsequent magnetic resonance examination is finally achieved. This increases the total examination time, which primarily also leads to a higher exposure of the patient.